It is desirable to control the explosive force deploying an automobile air bag so that the air bag's impact against a driver or passenger does not exceed safe thresholds. The amount of force used to deploy the air bag should be controlled as a function of seat position. For example, the air bag typically should be deployed with less force when the seat is close to, for example, the steering wheel than when the seat is distant from the steering wheel.
Automobile seats typically are fixed to a movable track or rail that can slide within or upon a second, stationary track or rail. As such, the seat can be moved along a path having terminal positions defined by the range of motion of the movable track relative to the stationary track. Using power or manual controls, the occupant can move the seat to a desired distance from the steering wheel.
The seat's proximity to the steering wheel can be determined, at least in part, by determining the position of the movable track affixed to the seat relative to the stationary track affixed to the floor using a conventional position sensor, such as a mechanical limit switch, an electronic switch, or a Hall effect sensor. Such sensors may be in communication with a controller, which controls air bag deployment based on the sensed position.
The stationary track typically is mounted to the automobile's floor and the movable track rides upon or within the stationary track. Debris, for example, toys, wads of aluminum foil, bits of food, coins and other foreign objects, often accumulates on the floor and under the seat, and may interfere with proper functioning of the seat's position sensor. Such debris may prevent detection or sensing of a target rail or object. If the seat track position sensor fails to function, an improper seat position indication may be processed by the air bag deployment controller, resulting in improper air bag deployment. As a result, the air bag may be deployed with an insufficient or excessive amount of force, resulting in injury or even death to the driver and/or passenger.
The inherent limitations of conventional sensors make them less than desirable for such applications. Conventional mechanical limit switches have proven unsuitable, given their moving parts are likely to break or wear out. Conventional electronic switches, such as capacitive switches have no moving parts to break or wear out. However, known electronic switches may respond to environments with Electro-Magnetic Interference (EMI) in unpredictable ways, and may not conform to increasingly rigid Electro-Magnetic Compatibility (EMC) standards. Hall effect sensors require precise placement of a magnet mounted on a target object and a sensor mounted in a position adapted to sense the target object. Both the target and the Hall effect sensor must be precisely mounted within tight tolerances to function reliably. Mounting the magnet and dedicated target have proven to be excessively difficult and therefore expensive.